We include in statistical model calculations the facts that in the nuclear multifragmentation process the fragments are produced within a given volume and have a finite size. The corrections associated with these constraints affect the partition modes and, as a consequence, other observables in the process. In particular, we find that the favored fragmenting modes strongly suppress the collective flow energy, leading to much lower values than those obtained from unconstrained calculations. For a given total excitation energy, this leads to a nontrivial correlation between the breakup temperature and the collective expansion velocity. In particular, we find that under some conditions, the temperature of the fragmenting system may increase as a function of this expansion velocity, contrary to what might be expected.